Twinkle, Twinkle, Little Star: Why Stars Appear to Twinkle - Skies at Night Are Big and Bright - Bad Astronomy: Misconceptions and Misuses Revealed, from Astrology to the Moon Landing “Hoax” - Philip Plait

Bad Astronomy: Misconceptions and Misuses Revealed, from Astrology to the Moon Landing “Hoax” - Philip Plait (2002)

Part III. Skies at Night Are Big and Bright

If we dare journey beyond the Moon looking for bad astronomy, we'll find a universe filled with weird things waiting to be misinterpreted.

Meteors are a major source of bad astronomy. When two eighteenth-century Yale scientists proposed that meteors were coming from outer space, one wag responded, "I would more easily believe that two Yankee professors would lie than that stones would fall from heaven." That wag was Thomas Jefferson. Thankfully, he stuck to other things like founding the University of Virginia (my alma mater) and running the country, and steered clear of astronomy.

If you go outside on a cloudless night, you might see a meteor or two if you're lucky. If you are not too close to a city and its accompanying light pollution, you'll see hundreds or even thousands of stars. Like meteors, that starlight has come a long way; even the closest known star is a solid 40 trillion kilometers away. And like meteors, those stellar photons end up as so much fodder for our human misunderstanding of the cosmos. Stars have color, they twinkle, they come in different brightnesses, and all of these characteristics are subject to clumsy misidentification.

Bad astronomy can often force the doomsayers out into the open, too. This happened in the years, months, and days leading up to the "Great" Planetary Alignment of May 2000. Last I checked, the world had not ended. Cries of doom always seem to pop up at solar eclipses as well. Long heralded as omens of the gods' ill favor, eclipses are actually one of the most beautiful sights the sky provides.

Finally, in this section we'll travel back in time and space to where it all began, the Big Bang. Something about contemplating the beginning of everything twists our already tangled minds, and descriptions of the Big Bang usually confuse the issue more than unravel it. The irony of the Big Bang, I suppose, is that it is even odder than our oddest theories could possibly suppose.

Chapter 9. Twinkle, Twinkle, Little Star: Why Stars Appear to Twinkle

"Twinkle, twinkle, little star, how I wonder what you are."

- Lyrics by Jane Taylor, music by Wolfgang Amadeus Mozart

"Twinkle twinkle, little planet, can't observe so better can it."

- The Bad Astronomer

was sitting at the observatory, waiting. It was 1990, and I was trying to make some observations as part of my master's degree work. The problem was the rain. It had poured that afternoon (not unusual for September in the mountains of Virginia), and I was waiting for the sky to clear up enough to actually get some good images.

After a few hours my luck changed, and the clouds broke up. Working quickly, I found a bright star and aimed the telescope there to focus on it. But try as I might, the image of the star on the computer screen never sharpened. I would move the focus in and out, trying everything, but no matter what I did the star image was hugely fuzzy.

So, I did what any astronomer locked in a small, dark room for three hours would do. I went outside and looked up.

The bright star I had chosen was high in the sky and twinkling madly. As I watched, it flashed spastically, sometimes even changing colors. I knew immediately why I couldn't get the star image sharp and crisp. The telescope wasn't to blame, our atmosphere was. I waited a couple of more hours, but the star refused to focus. I went home, resigned to try again the next night.

Who hasn't sat underneath the velvet canopy of a nighttime sky and admired the stars? So far away, so brilliant, so ... antsy?

Stars twinkle. It's very pretty. As you watch a star, it shimmers, it dances, it flickers. Sometimes it even changes color for a fraction of a second, going from white to green to red and back to white again.

But there, look at that star. Brighter than others, it shines with a steady, white glow. Why doesn't that one twinkle, too? If you wonder that aloud, a nearby person might smugly comment, "That's a planet. Planets don't twinkle, but stars do."

If you want to deflate them a little, ask them just why stars twinkle. Chances are, they won't know. And anyway, they're wrong. Planets can and do twinkle, as much as stars. It's just that twinkling rarely affects the way they look.

Having an atmosphere here on Earth has definite advantages like letting us breathe, fly paper airplanes, spin pinwheels on our bikes, and so on. But as much as we all like air, sometimes astronomers wish it didn't exist. Air can be a drag.

If the atmosphere were steady, calm, and motionless, then things would be fine. But it isn't. The air is turbulent. It has different layers, with different temperatures. It blows this way and that. And that turbulence is the root of twinkling.

One annoying property of air is that it can bend a light ray. This is called refraction, and you've seen it countless times. Light bends when it goes from one medium to another, like from air to water or vice versa. When you put a spoon in a glass of water, the spoon looks bent where the air meets the water. But, really, it's just the light coming out of the water and into the air that bends. If you've ever gone fishing in a stream armed with just a net, you've experienced the practical side of this, too. If you don't compensate for refraction, you're more likely to get a netful of nothing than tonight's dinner.

Light will bend when it goes from one part of the atmosphere to a slightly less dense part. For example, hot air is less dense than cooler air. A layer of air just over the black tar of a highway is hotter than the air just above it, and light going through these layers gets bent. That's what causes the blacktop ahead of you to shimmer on a summer's day; the air is refracting the light, making the highway's surface look like a liquid. Sometimes you can even see cars reflected in the layer.

Here on the ground, the air can be fairly steady. But, high over our heads things are different. A few kilometers up, the air is constantly whipping around. Little packets of air, called cells, blow to and fro up there. Each cell is a few dozen centimeters across and is constantly in motion. Light passing in and out of the cells gets bent a little bit as they blow through the path of that light.

That's the cause of twinkling. Starlight shines steady and true across all those light years to the Earth. If we had no atmosphere, the starlight would head straight from the star into our eyes.

But we do have air. When the starlight goes through our atmosphere, it must pass in and out of those cells. Each cell bends the light slightly, usually in a random direction. Hundreds of cells blow through the path of the starlight every second, and each one makes the light from the star jump around. From the ground, the size of the star is very small, much smaller than the cell of air. The image of the star, therefore, appears to jump around a lot, so what we see on the ground is the star appearing to dance as the light bends randomly. The star twinkles!

Astronomers usually don't call this twinkling, they call it seeing, a confusing holdover from centuries past, but like most jargon, it's stuck in the language. Astronomers determine how bad the seeing is on a given night by measuring the apparent size of a star. A star's image dances around so quickly that our eyes see this as a blurring into a disk of light. The worse the seeing is, the bigger the star looks. On a typical night, the seeing is a couple of arcseconds. For comparison, the Moon is nearly 2,000 arcseconds across, and the naked eye can just resolve a disk that is about 100 arcseconds across. The best seeing on the planet is usually a half an arcsecond, but it can be much larger, depending on how turbulent the air is.

Seeing also changes with time. Sometimes the air will suddenly grow calm for a few seconds, and the disk of a star will shrink dramatically. Since the light of the star gets concentrated into a smaller area, this lets you see fainter stars. I remember once sitting at the eyepiece of telescope for several minutes, looking for the very faint central star in a nebula. The star was just at the visibility limit of the telescope. Suddenly the seeing steadied up for a moment and the ghostly, pale-blue star snapped into my sight. Just as suddenly, the seeing went sour and the star disappeared. It was the faintest star I have ever seen with my own eyes, and it was amazing.

So why don't planets twinkle? Planets are big. Well, in reality they're a lot smaller than stars, but they are also a lot closer. Even the biggest star at night appears as a tiny dot to the world's best telescopes, but Jupiter is seen as a disk with just a pair of binoculars.

Jupiter is affected by seeing just as much as a star. But, since the disk of the planet is big, it doesn't appear to jump around. The disk does move, but it moves much less relative to its apparent size, so it doesn't appear to dance around like a tiny star does. Small features on the planet are blurred out, but the overall planet just sits there, more or less impervious to turbulence.

More or less. Under especially bad conditions even planets can twinkle. After thunderstorms the air can be very shaky, and if the planet is on the far side of the Sun the planet's disk will look particularly small, making it more susceptible to twinkling. But when a planet does twinkle, the seeing is incredibly bad, and observing is hopeless for that night.

Another way to increase twinkling is to observe near the horizon. When a star is just rising or setting, we are looking at it through more air because our atmosphere is curved. This means there are more cells between us and the star, and it can twinkle madly. Ironically, if you happen to be looking over a city, the air can be more stable. There are commonly smog layers over cities which stabilize the seeing, perhaps their only beneficial effect.

As it happens, different colors of light are refracted more easily than others. Blue and green, for example, bend much more than red. Sometimes, in really bad seeing, you can see stars change colors as first one color and then another is refracted toward you. Sirius is the brightest nighttime star, and it usually appears to be a steadily white color to the eye. But sometimes, when Sirius is low, it can flicker very dramatically and change colors rapidly. I have seen this myself many times; it's mesmerizing.

It can also lead to trouble. Imagine: You are driving along a lonely road at night and notice a bright object that appears to follow you. As you watch it flickers violently, going from bright to dim, and then you notice it's changing colors, from orange to green to red to blue! Could it be a spaceship? Are you about to be abducted by aliens?

No, you are a victim of bad astronomy. But the story sounds familiar, doesn't it? A lot of UFO stories sound like this. Stars appear to follow you as you drive because they are so far away. The twinkling of the star changes the brightness and the color, and imagination does the rest. I always smile when I hear a UFO tale like this one, and think that although it may not have been a UFO, it was definitely extraterrestrial.

Twinkling stars may inspire songs and poetry, but astronomers consider them an inconvenience. One of the reasons we build big telescopes is that they help increase our resolution of objects. Imagine two objects, one of them half the size of the other, but both smaller than the seeing on a given evening. Because of seeing, they will both get blurred out to the same size, and we cannot tell which object is larger. This puts a lower limit on how small an object we can observe and still accurately measure its size. Anything smaller than this lower limit will be blurred out, making it look bigger.

Even worse, objects that are close together will get blurred together by seeing, and we cannot distinguish them. This really puts the brakes on how small an object we can detect.

There are actually several ways to work around seeing. One way is to work over it. If you launch a telescope up over the atmosphere it won't be affected by seeing at all. That's the basic reason the Hubble Space Telescope was put into orbit in 1990. Without an atmosphere between it and the objects it studies, it has a better view than telescopes on the ground (for more, see chapter 22, "Hubble Trouble"). Hubble is not limited by seeing, and can usually resolve objects much better than its land-based brethren. The problem is that launching a telescope is very expensive, and can make a space telescope cost ten times as much as one built on the ground.

Another way around seeing is to take a lot of really short exposures of an object. If the exposure is fast enough, it freezes the image of the star before the turbulent air can blur it. It's like taking a fast exposure of a moving object. A one-second exposure of a race car is hopelessly blurred, but one taken at iho,ooo of a second will be clean and clear. A very fast exposure will show a clear image of the star, but the position of the star's image will jump around from exposure to exposure as the light bends. Astronomers can take hundreds or thousands of very short exposures of a star and then add the separate images together electronically, yielding detail that is impossible with longer exposures. This technique was used to get the first resolved image of a star other than the Sun. The red giant star Antares was the target, and the image, though blurry, was definitely resolved and not just a point of light.

The big disadvantage of this technique is that it only works for bright objects. A faint one won't show up in the short exposure times necessary. This severely limits the available targets and therefore the usefulness of the process.

There is a third technique that shows amazing promise. If the observer can actually measure just how the atmosphere is distorting a star's image, then the shape of the telescope mirror itself can be warped to compensate for it. This technique is called adaptive optics, or AO for short, because the optical system of the telescope can adapt to changes in the seeing. It's done by small pistons attached to rods, called actuators, located behind the telescope mirror. In some cases the rods push on the mirror, changing its shape, distorting the mirror just enough to correct for seeing changes. Another way is to use a collection of hexagonal mirrors that fit together like kitchen tiles, each with its own actuator. Little mirrors are much easier and less expensive to make than big ones, so many of the world's largest telescopes are designed this way.

The results are nothing less than incredible. The pictures above are from the Canada-France-Hawaii 3.6-meter telescope outfitted with AO. The image on the left is a picture of a binary star taken with the AO turned off. All we see is an elongated blur. But in the image on the right the AO is turned on, correcting for the seeing, and the two individual stars snap into focus.

The European Southern Observatory has several telescopes in Chile outfitted with adaptive optics. One is the Very Large Telescope, or VLT for short. The name isn't exactly poetic, but it does describe the huge, 8-meter, hexagonally segmented mirror pretty accurately. There are actually four such 'scopes, and with adaptive optics their images rival Hubble's. One of the only disadvantages of adaptive optics is the narrow field of view; only a small area of the sky can be seen in each exposure. As the technology improves, though, so will the area, and eventually these telescopes will routinely use AO for much larger chunks of the sky.

A close binary star pair may look like a blob of light when seen without adaptive optics (a), but is separated easily once the adaptive optics of the CFH telescope is switched on (b). Further image processing using computers can make the observation even better (c). The stars are separated by only about 0.3 arcseconds, or the apparent size of a quarter seen from a distance of almost 15 kilometers. (Image courtesy Canada France Hawaii Telescope Corporation, © 1996.)

The next time you're out on a clear night and the stars dance their dance, you can remember how even the simplest things like the twinkle of a star can have complicated origins, and how difficult it can sometimes be to work around them.

Or, you could just watch the stars twinkle. That's okay, too.